Method and Apparatus for Optical Sensing

ABSTRACT

An optical fiber distributed acoustic sensor system makes use of a specially designed optical fiber to improve overall sensitivity of the system by a factor in excess of 10. This is achieved by inserting into the fiber weak broadband reflectors periodically along the fiber. The reflectors reflect a small proportion of the light from the DAS incident thereon back along the fiber, typically in the region of 0.001% to 0.1%. To allow for temperate compensation to ensure that the same reflectivity is obtained if the temperature changes, the reflection bandwidth is relatively broadband. The reflectors are formed from a series of fiber Bragg gratings, each with a different center reflecting frequency, the reflecting frequencies and bandwidths of the gratings being selected to provide the broadband reflection. The reflectors are spaced at the desired spatial resolution of the optical fiber DAS.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.16/934,189, filed Jul. 21, 2020, which is a continuation of U.S.application Ser. No. 15/555,628, filed Sep. 5, 2017, which issued asU.S. Pat. No. 10,883,861 and which is a national stage application ofPCT International Application No. PCT/GB2016/050625, filed on Mar. 7,2016, which claims priority to United Kingdom Patent Application No.1503861.5, filed in the United Kingdom Intellectual Property Office onMar. 6, 2015, the disclosures of which are hereby incorporated byreference in their entireties.

TECHNICAL FIELD

Embodiments of the present invention relate to distributed optical fibresensors, and in particular in some embodiments to such sensors withreflective elements integrated into the sensing optical fiber.

BACKGROUND TO THE INVENTION AND PRIOR ART

Optical fiber based distributed sensor systems are finding manyapplications, in particular in the oil and gas industry for flowmonitoring and seismic detection, and in the security industry for areaor perimeter security monitoring, or monitoring along a long line suchas a pipeline or railway line. The present applicant, Silixa Ltd, ofElstree, London, markets two optical fiber distributed sensing systems,the Silixa® iDAS™ system, which is a very sensitive optical fiberdistributed acoustic sensor, and the Silixa® Ultima™ system, which is adistributed optical fiber based temperature sensor. Further details ofthe iDAS™ system are available at the priority date at silixa.com, andfurther details of the Ultima™ system are available at the priority dateat silixa.com. In addition, the present applicant's earlierInternational patent application WO 2010/136810 gives further technicaldetails of the operation of its distributed acoustic sensor system, theentire contents of which necessary for understanding the presentinvention being incorporated herein by reference.

The Silixa® iDAS™ system is presently class leading in terms of spatialresolution, frequency response, and sensitivity and is capable ofresolving individual acoustic signals with a spatial resolution of downto 1 m along the length of the fiber, at frequencies up to 100 kHz.However, it is always desirable to try and improve the performance interms of the any of the resolution, frequency response, or sensitivityparameters noted.

SUMMARY OF THE INVENTION

Embodiments of the invention provide an improved optical fiberdistributed acoustic sensor system that makes use of a speciallydesigned optical fiber to improve overall sensitivity of the system, insome embodiments by a factor in excess of 10. This is achieved byinserting into the fiber weak (by which we mean of low reflectivity)broadband reflectors periodically along the fiber. The reflectorsreflect only a small proportion of the light from the DAS incidentthereon back along the fiber, typically in the region of 0.001% to 0.1%,but preferably around 0.01% reflectivity per reflector. In addition, toallow for temperature compensation, the reflection bandwidth isrelatively broadband i.e. equal or greater than the region of +/−2 nm,preferably as large as +/−5 nm from the nominal laser wavelength. Thisprovides for temperature dependent reflectivity of the reflectors to beaccommodated, particularly where the reflectors are formed fromgratings, that are known to often exhibit temperature dependence of thereflected wavelength over a broad e.g. +/−2 nm bandwidth. In someembodiments the reflectors are formed from a series of fiber Bragggratings, each with a different center reflecting frequency, thereflecting frequencies and bandwidths of the gratings being selected toprovide the broadband reflection. In other embodiments a chirped gratingmay also be used to provide the same effect. In other embodiments ashort grating with low reflectivity and broad bandwidth may be writteninto the sensing fibre using femtosecond laser writing process. In someembodiments, the reflectors are spaced at the gauge length i.e. thedesired spatial resolution of the optical fiber DAS, in otherembodiments the reflectors are spaced at a distance calculated independence on the gauge length, for example as a fraction or multiplethereof.

In addition, some embodiments allow for either dual spatial resolutionoperation, or spatial resolution selectivity. These effects are obtainedby controlling the virtual timing characteristics between thereflections obtained from the reflectors as an optical pulse travelsalong the optical fiber. In particular, in one embodiment of theinvention an optical pulse is launched along the fiber and reflects fromthe reflectors in turn as it travels therealong. These reflections arereceived at a distributed acoustic sensing system, and are subject to aknown delay, to provide a delayed version of the reflections, which isthen interfered with the non-delayed version to obtain an output signal.The delay applied is referred to as the gauge length. The delayedversion can therefore be thought of as a virtual pulse that “follows”the actual pulse, but separated therefrom by the gauge length. Thedelay, or gauge length, between the actual pulse and the virtual pulsedefines the spatial resolution that is obtained from the system. Bycontrolling the gauge length with respect to the known spacing of thereflector portions, spatial resolution selectivity, or dual resolutionoperation may be obtained. In particular, the gauge length may becontrolled such that the effective pulse separation (i.e. the timingdifference or delay between the original pulse and the delayed pulse) isadjusted to encompass a desired pair of reflector portions, for exampleto alter the spatial resolution as desired. Dual spatial resolutionoperation is obtained by setting the gauge length to particular valuesthat mean that first and second sensing resolutions are obtainedalternately as a pulse travels along the fiber.

In view of the above, in some embodiments there is provided an opticalfiber distributed acoustic sensor system, comprising: an optical sourcearranged in use to produce optical signal pulses; an optical fiberdeployable in use in a sensing environment and arranged in use toreceive the optical signal pulses; and sensing apparatus arranged in useto detect light from the optical signal pulses reflected back along theoptical fiber and to determine acoustic signals incident on the opticalfiber in dependence on the reflected light; the system beingcharacterized in that the optical fiber comprises a plurality ofreflector portions regularly distributed along its length in at least afirst sensing region thereof.

Further features, embodiments, and advantages of the present inventionwill be apparent from the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, wherein like reference numerals refer to likeparts, and wherein:—

FIGS. 1, 2, 3 and 4 show schematic interferometer apparatus related toembodiments of the invention, comprising circulators and multiple fibrecouplers with different optical paths through the interferometers,Faraday-rotator mirrors and photodetectors;

FIGS. 5 and 6 show schematically how the interferometers can be cascadedaccording to embodiments of the invention in series and/or starconfigurations;

FIG. 7 shows schematically a sensor system that utilises aninterferometer for fast measurement of scattered and reflected lightfrom an optical fibre;

FIG. 8 shows schematically a distributed sensor system that utilises aninterferometer to generate a series of pulses each of differentfrequency and thereby allowing a different portion of the scatteredlight to interfere with another portion of the scattered light with aslight frequency shift resulting in a heterodyne beat signal;

FIG. 9 is a block diagram representing a data processing method;

FIG. 10 is a block diagram representing a method of calibrating aninterferometer;

FIG. 11 shows schematically a distributed sensor system where thespectrum of the light that is modulated using a fast optical modulator,that generators multiple frequency side bands with part of spectrumbeing selected using an optical filter.

FIG. 12A shows the spectrum of the light modulated and selected usingthe optical filter for the arrangement shown in FIG. 11 ;

FIG. 12B shows schematically a timing diagram for a method in accordancewith FIG. 11 ;

FIG. 13-1 is a diagram illustrating an embodiment fiber of the presentinvention;

FIG. 13-2 is a diagram illustrating a further embodiment fiber of thepresent invention;

FIG. 14-1 is a diagram illustrating the reflection bands of a series offiber Bragg gratings used in an embodiment of the invention;

FIG. 14-2 is a diagram illustrating the reflection bands of short fiberBragg gratings used in an embodiment of the invention;

FIG. 15 is a diagram of an alternative reflecting structure;

FIG. 16 is a diagram of another alternative reflecting structure;

FIGS. 17 and 18 illustrate mathematically the operation of embodimentsof the invention;

FIG. 19 is a series of sets of results illustrating performanceimprovements obtained using embodiments of the invention;

FIG. 20 is a diagram illustrating how pulses may be reflected to providedual-resolution operation in an embodiment of the invention;

FIGS. 21-1 A to J illustrate dual-resolution results be obtained by anembodiment of the invention;

FIG. 21-2 illustrates the dual-resolution where the laser pulse ischopped in the region where there is no overlap in the reflected region

FIG. 22 illustrates another embodiment where denser spacing is used toallow for resolution selectivity in an embodiment of the invention;

FIG. 23 is a diagram illustrating a processing chain for the dataderived from the dual-resolution system;

FIG. 24 is a diagram illustrating how the reflectors may just be in asubset of the fiber;

FIG. 25 is a diagram illustrating that the grating reflectors may havedifferent reflection bandwidths;

FIG. 26 is a diagram showing that the gratings may increase inreflectivity along the fiber;

FIG. 27 is a diagram illustrating how gratings may be formed within thecladding coupled to the core of the fiber, rather than in the coreitself;

FIG. 28 is a diagram showing how reflectors may be formed in the core ofa multi-mode fiber;

FIG. 29 is a diagram illustrating that the gratings may be provided injust one segment of fiber, and the pulse timing adjusted accordingly;

FIG. 30 is a diagram showing an alternative dual resolution arrangementwith different reflective spacing which can also be centered atdifferent wavelengths;

FIG. 31 is a diagram showing how reflectors may be formed in the core ofa multi-mode fiber;

FIGS. 32 and 33 are embodiments showing how the reflectors may be usedto provide backscatter free return channels in multimode and multi-corefibers;

FIGS. 34-1 and -2 (a) and (b) show how the resolution can be selected byusing gratings of different reflection bandwidths around the laserfrequency

FIGS. 35 and 36 are graphs illustrating properties of the reflectorsused in embodiments of the invention; and

FIG. 37 shows diagrams illustrating a further embodiment of theinvention.

OVERVIEW OF EMBODIMENTS

Embodiments of the invention provide an improved optical fiberdistributed sensor, and in some embodiments an optical fiber distributedacoustic sensor that improves on the Silixa® iDAS™ system described inWO 2010/136810, by improving the signal to noise ratio. This isaccomplished by using a sensing fiber having a number of weak,relatively broadband reflection points along the length thereof, spacedgenerally at the same distance as the gauge length, being the pathlength delay applied to the reflected pulse in one arm of theinterferometer of the DAS system, and which in turn relates to thespatial resolution obtained. Due to the weak reflectivity (around 0.01%reflectivity is envisaged), the reflection loss along the fiber issmall, and hence thousands of reflection point may be introduced. Forexample, for a sensing resolution of 10 m, 1000 reflection points givesan excess loss of just 0.4 dB, and a sensing length is obtained of 10km. The processing performed in the DAS system is substantiallyidentical to that performed on backscatter signals from along a standardfiber, but because there is a deliberate reflection back along the fiberrather than a scattering, a greater amount of reflected signal isreceived back at the DAS box, that is also more stable, both factors ofwhich contribute to the increase in signal to noise performance. Aspecific aspect that helps to increase SNR further is that because thereflection points are fixed along the fiber then 1/f noise that is dueto the fundamental nature of random backscattering is reduced to anunmeasurable level. This helps reduce the noise floor of the signal ofthe processed signal. Hence, by increasing the optical signal level incombination with the reduction in 1/f noise, total signal to noise ratiois increased. Tests of the technique show that an improvement in signalto noise ratio in excess of a factor of 10 is achieved.

Regarding the nature of reflection points, in some embodiments a seriesof Fiber Bragg Gratings (FBGs) are used for each reflection point, witha different peak reflection wavelength but with overlapping reflectionbandwidths, the gratings being written into the fiber next to eachother, separated by a small amount, of the order of 5 to 15 mm, andpreferably around 10 mm. Where 5 gratings are used with a 10 mmseparation between them, the total length of each reflection point isaround 45 mm, and the total reflection bandwidth allowing for theoverlapping reflection bandwidths of the individual gratings is around+/−2 nm, although in some embodiments it can be as wide as at least +/−5nm. In other embodiments ideally a single, relatively weak broadbandreflector would be used; for example a chirped grating or a short,broadband, weakly reflecting mirror less than 1 mm and typically 100 □min length. Further embodiments are described below.

The use of reflection points along the fiber also opens up otherpossibilities, particularly concerning the spatial resolution of theDAS. For example, in some embodiments a simultaneous dual-resolutionarrangement can be provided, by selection of appropriate gauge lengthand pulse width with respect to the spacing of the reflector portionsalong the fiber. For example, for a given reflector spacing L, providedthe pulse width is less than L, for example around 0.75L, and furtherprovided that the gauge length, i.e. the difference in length betweendifferent arms of the interferometer in the DAS, which in turn relatesto the spatial resolution, is chosen such that the reflected light andthe delayed version thereof in the interferometer have beenconsecutively reflected from neighbouring reflection points and thennon-neighbouring reflection points, then multi-resolution performancewill be successively obtained. For example, where L is 10 m, pulse widthis 7.5 m, and gauge length (effective virtual pulse separation in theinterferometer corresponding to interferometer path length difference)is 15 m, then alternating 10 m and 20 m resolution performance isobtained as the pulses travel along the fiber.

In other embodiments the control of pulse timing characteristics withrespect to reflector separation allows for resolution selectivity. Inthese embodiments, the reflector separations can be smaller than theinitial gauge length, such that a first spatial resolution is obtained,but by then reducing the gauge length to match the smaller pitch of thereflectors then a second, improved, resolution is obtained. Providing adenser spatial distribution of reflectors therefore allows selectivespatial resolution from the same fiber. In preferred embodiments, thereflectors are spaced at half the gauge length i.e. at half the desiredspatial resolution of the optical fiber DAS.

In view of the above, and given the fact that the distributed acousticsensor can be identical to those described previously, in the detaileddescription of embodiments given below a distributed acoustic sensor asdescribed in WO 2010/136810 is described for completeness with respectto FIGS. 1 to 12 , and then further description is undertaken of thefiber provided by embodiments of the present invention, and how thedistributed acoustic sensor systems of the described embodiments may befurther adapted to accommodate use of the described fiber as the sensingfiber to obtain the improved sensitivity and spatial resolutionenhancements.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a first embodiment, generally depicted at 100, of aninterferometer for measuring the optical amplitude, phase and frequencyof an optical signal. The incoming light from a light source (not shown)is preferably amplified in an optical amplifier 101, and transmitted tothe optical filter 102. The filter 102 filters the out of band AmplifiedSpontaneous Emission noise (ASE) of the amplifier 101. The light thenenters into an optical circulator 103 which is connected to a 3×3optical coupler 104. A portion of the light is directed to thephotodetector 112 to monitor the light intensity of the input light. Theother portions of light are directed along first and second opticalpaths 105 and 106, with a path length difference (109) between the twopaths. The path length difference therefore introduces a delay into onearm 105 of the interferometer, such that the light that is reflectedback for interference at any one time in that arm 105 is from a pointcloser along the fiber than the light available for interference in theother arm 106. This difference in length between the arms of theinterferometer relates to (but is not quite equal to) the spatialresolution obtained, and is referred to herein as the gauge length.Faraday-rotator mirrors (FRMs) 107 and 108 at the ends of theinterferometer arms reflect the light back through the first and secondpaths 105 and 106, respectively. The Faraday rotator mirrors provideself-polarisation compensation along optical paths 105 and 106 such thatthe two portions of light reflected from the FRMs efficiently interfereat each of the 3×3 coupler 104 ports. The optical coupler 104 introducesrelative phase shifts of 0 degrees, +120 degrees and −120 degrees to theinterference signal, such that first, second and third interferencesignal components are produced, each at a different relative phase.

First and second interference signal components are directed by theoptical coupler 104 to photodetectors 113 and 114, which measure theintensity of the respective interference signal components.

The circulator 103 provides an efficient path for the input light andthe returning (third) interference signal component through the sameport of the coupler 104. The interference signal component incident onthe optical circulator 103 is directed towards photodetector 115 tomeasure the intensity of the interference signal component.

The outputs of the photodetectors 113, 114 and 115 are combined tomeasure the relative phase of the incoming light, as described in moredetail below with reference to FIGS. 7 and 9 .

Optionally, frequency shifters 110 and 111 and/or optical modulator 109may be used along the paths 105 and 106 for heterodyne signalprocessing. In addition, the frequency shift of 110 and 111 may bealternated from f1, f2 to f2, f1 respectively to reduce anyfrequency-dependent effect between the two portions of the lightpropagating through optical paths 105 and 106.

The above-described embodiment provides an apparatus suitable for fastquantitative measurement of perturbation of optical fields, and inparticular can be used for distributed and multiplexed sensors with highsensitivity and fast response times to meet requirements of applicationssuch as acoustic sensing.

FIG. 7 shows an application of the interferometer of FIG. 1 to thedistributed sensing of an optical signal from an optical system 700. Itwill be apparent that although the application is described in thecontext of distributed sensing, it could also be used for point sensing,for example by receiving reflected light from one or more point sensorscoupled to the optical fibre.

In this embodiment 700, light emitted by a laser 701 is modulated by apulse signal 702. An optical amplifier 705 is used to boost the pulsedlaser light, and this is followed by a band-pass filter 706 to filterout the ASE noise of the amplifier. The optical signal is then sent toan optical circulator 707. An additional optical filter 708 may be usedat one port of the circulator 707. The light is sent to sensing fibre712, which is for example a single mode fibre or a multimode fibredeployed in an environment in which acoustic perturbations are desiredto be monitored. A length of the fibre may be isolated and used as areference section 710, for example in a “quiet” location. The referencesection 710 may be formed between reflectors or a combination of beamsplitters and reflectors 709 and 711.

The reflected and the backscattered light generated along the sensingfibre 712 is directed through the circulator 707 and into theinterferometer 713. The detailed operation of the interferometer 713 isdescribed earlier with reference to FIG. 1 . In this case, the light isconverted to electrical signals using fast low-noise photodetectors 112,113, 114 and 115. The electrical signals are digitised and then therelative optical phase modulation along the reference fibre 710 and thesensing fibre 712 is computed using a fast processor unit 714 (as willbe described below). The processor unit is time synchronised with thepulse signal 702. The path length difference (109) between path 105 andpath 106 defines the spatial resolution. The photodetector outputs maybe digitised for multiple samples over a given spatial resolution. Themultiple samples are combined to improve the signal visibility andsensitivity by a weighted averaging algorithm combining thephotodetector outputs.

It may be desirable to change the optical frequency of the lightslightly to improve the sensitivity of the backscattered or reflectedsignals. The optical modulator 703 may be driven by a microwavefrequency of around 10-40 GHz to generate optical carrier modulationsidebands. The optical filter 708 can be used to select the modulationsidebands which are shifted relative to the carrier. By changing themodulation frequency it is possible to rapidly modulate the selectedoptical frequency.

Data Processing

FIG. 9 schematically represents a method 1100 by which the optical phaseangle is determined from the outputs of the photodetectors 113, 114,115. The path length difference between path 105 and path 106 definesthe spatial resolution of the system. The photodetector outputs may bedigitised for multiple samples over a given spatial resolution, i.e. theintensity values are oversampled. The multiple samples are combined toimprove the signal visibility and sensitivity by a weighted averagingalgorithm combining the photo-detector outputs.

The three intensity measurements I₁, I₂, I₃, from the photodetectors113, 114, 115 are combined at step 1102 to calculate the relative phaseand amplitude of the reflected or backscattered light from the sensingfibre. The relative phase is calculated (step 1104) at each samplingpoint, and the method employs oversampling such that more data pointsare available than are needed for the required spatial resolution of thesystem.

Methods for calculating the relative phase and amplitude from threephase shifted components of an interference signal are known from theliterature. For example, Zhiqiang Zhao et al. in “Improved DemodulationScheme for Fiber Optic Interferometers Using an Asymmetric 3×3 Coupler”,J. Lightwave Technology, Vol. 13, No. 1 1, November 1997, pp. 2059-2068and also U.S. Pat. No. 5,946,429 describe techniques for demodulatingthe outputs of 3×3 couplers in continuous wave multiplexingapplications. The described techniques can be applied to the time seriesdata of the present embodiment.

For each sampling point, a visibility factor V is calculated at step1106 from the three intensity measurements I₁, I₂, I₃, from thephotodetectors 113, 114, 115, according to equation (1), for each pulse.

V=(I ₁ −I ₂)²+(I ₂ −I ₃)²+(I ₃ −I ₁)²  Equation (1)

At a point of low visibility, the intensity values at respective phaseshifts are similar, and therefore the value of V is low. Characterisingthe sampling point according the V allows a weighted average of thephase angle to be determined (step 1108), weighted towards the samplingpoints with good visibility. This methodology improves the quality ofthe phase angle data 1110.

Optionally, the visibility factor V may also be used to adjust (step1112) the timing of the digital sampling of the light for the maximumsignal sensitivity positions. Such embodiments include a digitiser withdynamically varying clock cycles, (which may be referred to herein as“iclock”). The dynamically varying clock may be used to adjust thetiming of the digitised samples at the photodetector outputs for theposition of maximum signal sensitivity and or shifted away frompositions where light signal fading occurs.

The phase angle data is sensitive to acoustic perturbations experiencedby the sensing fibre. As the acoustic wave passes through the opticalfibre, it causes the glass structure to contract and expand. This variesthe optical path length between the backscattered light reflected fromtwo locations in the fibre (i.e. the light propagating down the twopaths in the interferometer), which is measured in the interferometer asa relative phase change. In this way, the optical phase angle data canbe processed at 1114 to measure the acoustic signal at the point atwhich the light is generated.

In preferred embodiments of the invention, the data processing method1100 is performed utilising a dedicated processor such as a FieldProgrammable Gate Array.

Sensor Calibration

For accurate phase measurement, it is important to measure the offsetsignals and the relative gains of the photo-detectors 113,114 and 115.These can be measured and corrected for by method 1200, described withreference to FIG. 10 .

Each photodetector has electrical offset of the photodetectors, i.e. thevoltage output of the photodetector when no light is incident on thephotodetector (which may be referred to as a “zero-light level” offset.As a first step (at 1202) switching off the incoming light from theoptical fibre and the optical amplifier 101. When switched off, theoptical amplifier 101 acts as an efficient attenuator, allowing nosignificant light to reach the photodetectors. The outputs of thephotodetectors are measured (step 1204) in this condition to determinethe electrical offset, which forms a base level for the calibration.

The relative gains of the photodetectors can be measured, at step 1208,after switching on the optical amplifier 101 while the input light isswitched off (step 1206). The in-band spontaneous emission (i.e. theAmplified Spontaneous Emission which falls within the band of thebandpass filter 102), which behaves as an incoherent light source, canthen be used to determine normalisation and offset corrections (step1210) to calibrate the combination of the coupling efficiency betweenthe interferometer arms and the trans-impedance gains of thephotodetectors 113, 114 and 115. This signal can also be used to measurethe signal offset, which is caused by the in-band spontaneous emission.

Conveniently, the optical amplifier, which is a component of theinterferometer, is used as in incoherent light source without arequirement for an auxiliary source. The incoherence of the source isnecessary to avoid interference effects at the photodetectors, i.e. thecoherence length of the light should be shorter than the optical pathlength of the interferometer. However, for accurate calibration it ispreferable for the frequency band of the source to be close to, orcentred around, the frequency of light from the light source. Thebandpass filter 102 is therefore selected to filter out light withfrequencies outside of the desired bandwidth from the AmplifiedSpontaneous Emission.

When used in a pulsed system, such as may be used in a distributedsensor, the above-described method can be used between optical pulsesfrom the light source, to effectively calibrate the system during use,before each (or selected) pulses from the light source withsubstantively no interruption to the measurement process.

Variations to the above-described embodiments are within the scope ofthe invention, and some alternative embodiments are described below.FIG. 2 shows another embodiment, generally depicted at 200, of a novelinterferometer similar to that shown in FIG. 1 but with an additionalFaraday-rotator mirror 201 instead of photodetector 112. Like componentsare indicated by like reference numerals. In this case the interferencebetween different paths, which may have different path length, can beseparated at the three beat frequencies f₁, f₂ and (f₂−f₁). Thearrangement of this embodiment has the advantage of providing additionalflexibility in operation, for example the different heterodynefrequencies can provide different modes of operation to generatemeasurements at different spatial resolutions.

FIG. 3 shows another embodiment of a novel interferometer, generallydepicted at 300, similar to the arrangement of FIG. 1 , with likecomponents indicated by like reference numerals. However, thisembodiment uses a 4×4 coupler 314 and an additional optical path 301,frequency shifter 304, phase modulator 303, Faraday-rotator mirror 302and additional photo-detector 308. Additionally, the first optical path105 includes a frequency shifter 305 and the second optical path 106includes a frequency shifter 306 and phase modulator 307. In this casethe interference between different paths, which may have different pathlength differences, can be separated at the three beat frequencies(f2−f1), (f3−f2) and (f3−f1). Alternatively, the Faraday-rotator mirror302 may be replaced by an isolator or a fibre matched end so that nolight is reflected through path 301, so only allowing interferencebetween path 105 and 106.

The 4×4 optical coupler of this arrangement generates four interferencesignal components at relative phase shifts of −90 degrees, 0 degrees, 90degrees, 180 degrees.

FIG. 4 shows another embodiment of the interferometer. In this case anadditional path is introduced in the interferometer by inserting aFaraday-rotator mirror 402 instead of the photo-detector 112.

In all of the above-described embodiments, optical switches may be usedto change and/or select different combinations of optical path lengthsthrough the interferometer. This facilitates switching between differentspatial resolutions measurements (corresponding to the selected pathlength differences in the optical path lengths).

FIGS. 5 and 6 show examples of interferometer systems 500, 600 arrangedfor used in cascaded or star configurations to allow the measuring ofthe relative optical phase for different path length differences. InFIG. 5 , three interferometers 501, 502, 503 having different pathlength differences (and therefore different spatial resolutions) arecombined in series. In FIG. 6 , four interferometers 602, 603, 604 and605 having different path length differences (and therefore differentspatial resolutions) are combined with interferometers 602, 603, 604 inparallel, and interferometers 603 and 605 in series. In FIG. 6, 601 is a3×3 coupler, used to split the light between the interferometers.Arrangement 600 can also be combined with wavelength divisionmultiplexing components to provide parallel outputs for differentoptical wavelengths.

The embodiments described above relate to apparatus and methods for fastquantitative measurement of acoustic perturbations of optical fieldstransmitted, reflected and or scattered along a length of an opticalfibre. Embodiments of the invention can be applied or implemented inother ways, for example to monitor an optical signal generated by alaser, and/or to monitor the performance of a heterodyne signalgenerator, and to generate optical pulses for transmission into anoptical signal. An example is described with reference to FIG. 8 .

FIG. 8 shows a system, generally depicted at 800, comprising aninterferometer 801 in accordance with an embodiment of the invention,used to generate two optical pulses with one frequency-shifted relativeto the other. The interferometer receives an input pulse from a laser701, via optical circulator 103. A 3×3 optical coupler 104 directs acomponent of the input pulse to a photodetector, and components to thearms of the interferometer. One of the arms includes a frequency shifter110 and an RF signal 805. The interference between the two pulses ismonitored by a demodulator 802. The light reflected by Faraday-rotatormirrors 107 and 108 is combined at the coupler 809 using a delay 803 tomatch the path length of the interferometer, so that the frequencyshifted pulse and the input pulse are superimposed. The coupler 809introduces relative phase shifts to the interference signal, andinterferometer therefore monitors three heterodyne frequency signalcomponents at relative phase shifts. The optical circulator 103 passesthe two pulses into the sensing fibre.

In this embodiment, the reflected and backscattered light is notdetected by an interferometer according to the invention. Rather, thereflected and backscattered light is passed through an optical amplifier804 and an optical filter 806 and are then sent to a fast, low-noisephotodetector 807. The electrical signal is split and thendown-converted to baseband signals by mixing the RF signal 805 atdifferent phase angles, in a manner known in the art. The electricalsignals are digitised and the relative optical phase modulation at eachsection of the fibre is computed by combining the digitised signalsusing a fast processor 808.

FIG. 11 shows another embodiment of apparatus for point as well asdistributed sensors. In this case the modulation frequency 704 of theoptical modulator 703 is switched from f1 to f2 within the optical pulsemodulation envelope.

The optical filter 708 selects two modulation frequency sidebands1202/1203 and 1204/1205 generated by the optical modulator as indicatedin FIG. 12 . The frequency shift between first order sidebands 1202 and1203 is proportional to the frequency modulation difference (f2−f1)whereas the frequency shift between 2^(nd) order sidebands 1204 and 1205is proportional to 2(f2−f1). Therefore, the photo-detector output 806generates two beat signals, one of which is centred at (f2−f1) and theother at 2(f2−f1). Using the demodulator 901, the relative optical phaseof the beat signals can be measured independently. The two independentmeasurements can be combined to improve the signal visibility, thesensitivity and the dynamic range along the sensing fibre.

FIG. 12A shows the modulation spectrum of the light and the selection ofthe sidebands referred to above.

FIG. 12B shows the original laser pulse 1206 with pulse width of T atfrequency fo which is modulated at frequency f1, f2 and f3 during aperiod T1, T2 and T3, respectively. The delay between T1, T2 and T3 canalso be varied. One or more modulation sidebands is/are selected withthe optical filter 708 to generated a frequency shifted optical pulsesthat are sent into the fibre. The reflected and/or backscatter signals(709, 710, 711 and 712) from the fibre from is directed to aphotodetector receive via a circulator 707. The reflected and orbackscatter light from different pulses mix together at thephotodetector output to generate heterodyne signals such (f2−f1),(f3−f1), (f3−f2), 2(f2−f1), 2(f3−f1) and 2(f3−f2). Other heterodynesignals are also generated but (2f2−f1), (2f3−f1), (2f1−f2), (2f1−f3),(2f3−f1) and (2f3−f2) are also generated at much higher frequencies. Theheterodyne signal are converted down to base band in-phase andquadrature signals. The in-phase and quadrature signals are digitise bya fast analogue to digital convertors and the phase angle is computedusing fast digital signal processor.

As noted, the above described embodiments correspond to those alreadypublished in our previous International patent application no WO2010/136810, and relate to various versions of an optical fiberdistributed acoustic sensor that form the basis for embodiments of thepresent invention. As previously explained in the overview sectionabove, embodiments of the present invention make use of any of thepreviously published arrangements with a modified fiber that includesspecific reflection points along its length, spaced in dependence on theintended spatial resolution (strictly speaking to the gauge length) ofthe DAS, and also optionally with some additional signal processingenhancements, to significantly increase the sensitivity of the overalloptical fiber sensing system thus obtained. Further details are givennext.

The performance of a fiber optic distributed acoustic sensor (DAS), formost applications, is limited by the system's acoustic signal to noiseratio (SNR). Improving the acoustic SNR can lead to, for example,quantification of flow, seismic and leak signals which are otherwiseunmeasurable. The acoustic SNR of a DAS in turn depends upon the DASoptical SNR, which is the relationship of the magnitude of the opticalsignal and the associated detection noise. The optical SNR, and so theacoustic SNR, is optimised by maximising the amount of optical signalreturning from the optical fibre.

The returning optical signal can be maximised using a number oftechniques, including using a shorter wavelength source light (shorterwavelengths scatter more) and using a fibre with a large scatteringcoefficient or a large capture angle. Herein we describe a techniqueusing introduced reflection points at pre-determined positions in thefibre. These reflection points should have the functionality ofpartially-reflective mirrors—ideally they should reflect a small amountof light (typically less than 0.1%) across a relatively large bandwidth(e.g. of the order of >2 nm) and transmit the remainder. Even with sucha small reflectivity, the amplitude of reflected light will still bemore than an order of magnitude larger than that of the naturallybackscattered light over the same spatial interval. The use ofreflectors therefore has an advantage over other techniques, such asusing a higher scattering coefficient, in that all of the light lostfrom the transmission is reflected back towards the DAS rather thanscattered in all directions.

As well as increasing the optical signal, the use of reflection pointsgives another significant benefit to the DAS noise characteristics. Thisis because, when using standard fibres, a DAS is typically subject to1/f noise, meaning that the acoustic noise for low frequencies(particularly below 10 Hz) is significantly higher than the noise athigher frequencies. The existence of 1/f noise is fundamental to therandom nature of backscattering characteristics and dominates the DASperformance for low frequency measurements, which constitute a majorpart of the DAS applications. When using reflection points, however,reflection characteristics are fixed, rather than random, and this hasthe effect of reducing the 1/f noise to an unmeasurable level. Thecontrolled scattering also results in a uniform noise floor, bothspatially and temporally, whereas random scattering inherent produces anoise floor characteristic which typically varies rapidly in distanceand slowly in time. In addition, the fixed reflection characteristicsresult in a more stable measured acoustic amplitude than is achievedusing backscattering.

In an ideal embodiment each reflection point along the fiber would be aweak mirror—that is a reflection point would reflect all wavelengths oflight equally, with a constant reflection coefficient. Typically, thereflection coefficient would be around 0.01% (which is around 100×morelight than is backscattered per metre of fibre), meaning that eachmirror reflects 0.01% and transmits 99.99% of the incident light.However, a range of reflection coefficients would be acceptable, forexample ranging for example from 0.002% to 0.1%, depending on designconsiderations. Generally, a smaller reflection coefficient will lead toless light reflected, and hence a smaller performance improvement, butwill allow for greater theoretical range, whereas a higher reflectioncoefficient will reflect more light and hence provide greater signal tonoise ratio, but may impact the sensing range along the fiber,particularly for finer spatial resolutions where the reflection pointsare closer together. Where a reflection coefficient of 0.01% is used,then due to the low loss at each reflection point, it is practical toinsert many 100s of such weak mirrors into the fibre without introducingsignificant optical losses. For example, 1000 reflection points wouldintroduce an excess loss of just 0.4 dB (equivalent to the loss of 2 kmof standard optical fibre).

The reflection points are typically spaced at the same distance as thespatial resolution (“gauge length”) of the DAS. This means that, if theDAS spatial sensing resolution is 10 m, 1000 reflection points can beused to make up a total sensing length of 10 km. In some embodiments theDAS needs no modification to make it compatible with the fibrecontaining the reflection points—conceptually, the DAS treats the newfibre the same as standard fibre (albeit with a higher scatteringcoefficient). In other embodiments, however, the DAS signal processingcan be optimised for use with this fibre by making use of the fact thatnow all sensing positions between each pair of reflection points measurethe same signal. This means, for example, we can measure many positionsbetween reflection points and then average the signals from thesepositions to improve the SNR.

In addition, the increased signal to noise ratio can be used tosignificantly improve the spatial resolution of the DAS while stillmaintaining an acceptable noise performance. For example, whereas a DASusing backscatter requires a gauge length of around 1 m to achieve anacceptable SNR for most applications, using reflectors, an acceptableSNR can be achieved with a gauge length of around 5 cm. Such animprovement in spatial resolution allows the accurate measurement ofultrasonic signals and, for example, the tracking of features associatedwith short length scales, such as eddies in pipes. It may be necessaryto use a phase correlation arrangement with a narrow detection bandwidthto achieve sufficient signal-to-noise performance. There can also beadvantages in long range applications such as leak detection, pipelinesand subsea.

Another feature is to measure very small temperature variations along asection of a pipe caused by fluid flow down to less than m° K and up tofew Hertz. The high resolution temperature measurement can be used tomeasure the fluid flow by observing the propagation of the exchange ofthe turbulent thermal energy.

Note, as the DAS configuration may be identical for measuring on eitherstandard fibre or fibres with reflection points it is possible toperform a hybrid measurement, where the DAS simultaneously measures onboth fibre types. Here, for example, the reflection points could bepositioned at strategic positions where more sensitivity is required(for example to measure flow) whereas the rest of the fibre, which isunmodified, is used for measurements, such as seismic, where morecoverage and less sensitivity is required.

Although as noted above the ideal reflection point would be a weakmirror—that is that it would reflect all wavelengths of light equally,with a constant reflection coefficient—the most suitable currenttechnology to form the reflection point is the Fibre Bragg Grating(FBG). A FBG is usually designed as either an optical filter or as asensing element where the peak reflection wavelength of the grating isused to determine the grating spacing and hence the strain ortemperature of the FBG. FBGs can be written into an optical fiber usingfemtosecond laser writing processes. In particular, FBGs can now bewritten directly into an optical fibre as the fibre is drawn, and beforethe fibre is coated, making it commercially and technically practicableto produce a fibre with 1000s of FBGs. Additionally, FBGs can beembedded into an optical fiber by changing its refractive index usingfemtosecond laser writing processes or written by UV laser during fiberdrawing.

One drawback with commercial FBGs is that they are generally designed tomaximise peak reflection, and to selectively reflect a particularwavelength (which may change with temperature or strain). In the presentembodiments, on the other hand, we want the opposite characteristics—alow level of reflection over a broad wavelength range such that ourlaser light is constantly reflected as the temperature of the FBGchanges (for example as the cable is deployed down an oil well).

In order to address this issue in the prototype embodiments we havetested to date, the change in peak reflection wavelength withtemperature is dealt with by using five overlapping (in wavelength) FBGsat each reflection point. Each FBG in this example is around 1 mm inlength, with a separation of 10 mm between FBGs, meaning the totallength of each reflection point is around 45 mm. The overall reflectionbandwidth is around +/−2 nm. It was found though that this configurationis not ideal. This is because the overlap (in wavelength) of thegratings required to ensure we get reflection over the whole bandwidthleads to interference between the FBGs at each reflection point when theDAS laser is in the overlapping range (see FIG. 14 ). For this reason,in other embodiments we propose to change the FBG design to either of:

A single broadband grating. Generally, a broadband grating is also aweak grating, which is good in this application where a weak reflectionand large transmission is desired. Generally, an optimum FBG for thisapplication is what the industry would usually think of as a “badgrating”, in that research is geared towards narrowing bandwidth andincreasing the reflectivity. In contrast, our ideal grating may provideweak, broadband reflectivity, and be written into the fiber usingfemtosecond laser writing techniques.

A “chirped” grating. This has a varying reflection wavelength along thelength of the grating. This allows broadband reflectivity without theinterference issues we experience. In this case, again a weakreflectance is all that is required, across the reflection bandwidth.

Crosstalk

The use of reflection points introduces crosstalk caused by multiplereflections between the reflection points. These multiple paths willlead to an ambiguity in the location of a proportion of the opticalsignal (crosstalk). Our modelling suggests that this will not be a majorissue for our target applications, provided the sum of thereflectivities of the reflection points does not exceed ˜10%. Forexample, this condition allows the equivalent of 1000 reflection pointseach with a reflectivity of 0.01%.

If needed, novel architectures may be used to reduce the crosstalk, forexample by using angled gratings, such as shown in FIG. 15 or by using acombination of couplers and mirrors, such as shown in FIG. 16 . In moredetail, FIG. 15 illustrates the use of an angled grating 1510 extendingacross the fiber. The grating includes a right-angled elbow section1530, that is arranged such that it receives light passing along thefiber from a first direction, and reflects it back in the oppositedirection, via two substantially 90 degree reflections from the grating.That is, the light from the first direction is incident on the “inside”edge of the elbow, such that it is then reflected back in the directionit came.

Conversely, light passing along the fiber from a second direction,opposite to the first direction, is incident on the “outside” edge ofthe elbow, such that it then reflects off the outside edge at 90 degreesto its original direction, and is then scattered out of the fiber. Suchan arrangement should reduce cross-talk by preventing multiplereflections between different nearby gratings.

Another technique to reduce crosstalk is to increase the reflectivityover distance. This works because the nearer markers contribute most tothe crosstalk. Such an arrangement also has the advantage that thereflectivity profile can be chosen to compensate for loss in the fibre,so giving an equal SNR along the fibre length.

FIG. 16 illustrates an alternative arrangement where a coupler componentis used to couple a small proportion of the light into another fiber,that is then coupled to a mirror. Here, preferably the mirror is fully100% reflective, and the coupling coefficient of the coupler iscontrolled so as to couple only a small amount (e.g. 0.01%, or some suchvalue, as discussed above) of the incident light towards the mirror, sothat the same overall weak reflection that is desired as described aboveis obtained.

FIGS. 17 and 18 illustrate the concept of embodiments of the inventionnumerically. In FIG. 17A, a typical DAS scenario of the prior art isconsidered. Here, the main DAS limitation is the scattering light lossesas only a small part of the scattered light (Θ=10 ⁻³) comes back alongthe fiber, and in reality often an even smaller number (Θ=104) can bepractically used. In contrast, in a typical time domain interferometricmultiplexed setup, such as shown in FIG. 17B (and taken from Kersey etal. Cross talk in a fiber-optic sensor array with ring reflectors,Optics Letters, Vol 14, No. 1 Jan. 1 1989) up to third of photons may beused for measurement purposes.

Therefore, in order to increase the amount of light returned along thefiber available for sensing purpose in the type of DAS consideredherein, as discussed above consider the intermediate setup shown in FIG.17C, where backscattering is enhanced by deliberately placing weak,broadband reflectors at points along the fiber. As modelled here, thereflectors are fiber Bragg gratings (FBGs), as discussed previously. Todeliver a crosstalk of less than 1% the total reflection along the wholelength of fiber should preferably be less than 10% (RN<0.1) and withsuch constraints it is possible to cover 3 km of fiber by gratings witha 10 m period. Such a system can then deliver a shot noise more than 10times better than current DAS systems, as discussed earlier.

Considering the issue of crosstalk again, a crosstalk estimation for theprevious DAS arrangements (shown as the set of equations FIG. 18A) givesoptimistic results based on incoherent addition of back-reflected light,when between 100 and 1000 times more photons can be involved in acousticmeasurements with respect to ideal Rayleigh backscattering (NR˜100Θ,where Θ=10⁻³ is stereo angle of scattering). Contrary if we suppose thatall light is coherent and optical fields should be added instead ofintensities then the result is quite pessimistic, see FIG. 18 B. In thiscase a set of low contrast Fabry-Perot interferometers such as describedin the Kersey paper ibid. is not more effective than backscattering (forthe same crosstalk 1%) as NR˜Θ. The Kersey findings are presented atFIG. 18C; the result is intermediate NR˜10Θ only moderately better thanRayleigh. This result can be an explanation why such simple acousticantennae were not popular for 25 years of hydrophone time domainmultiplexing. Nevertheless near 10 times SNR improvement can bepotentially achieved with a real system keeping in mind real DASvisibility and losses.

FIG. 19 illustrates the results of testing the concept on a fiber modelwith 4 gratings with reflectivity 0.001% separated by 10 m which areclearly visible under short pulse (10 ns) illumination (see FIG. 19A).As far as a 10 m resolution DAS was used we can see also additionalreflections delayed by the gauge length (the path length differencebetween the arms 105 and 106 of the interferometer in the DAS, and whichsets the initial spatial resolution of the DAS), so interference wasinside 3 low contrast Fabry-Perot interferometers. An acoustic signalwas applied for 2 of them only, as visible on the waterfall graphs (themiddle graphs) measured with 1 m sampling. Here, it can be seen that theacoustic signal is applied repeatedly every 30 time samples or so at adistance of between 50 m and 70 m along the fiber. A longer opticalpulse (70 ns) can generate more signal as is clear from FIG. 19B. Thered horizontal line in the spectrum corresponds to the acousticmodulation zone (i.e. where the acoustic signal was applied); opticalcrosstalk to the third interferometer located between 70 m and 80 m isnegligible.

Finally a tiny reflection was modelled which was only 10 times biggerthan backscattering level (R=0.0001%), see FIG. 19C. Nevertheless theDAS signal improvement was even more than 3 times (as can be expectedfrom a shot noise), partly because of good visibility. The modellingresults confirm that the SNR improvement can be even more than 10 times.

One interesting advantage of the using regularly spaced reflectors inthe sensing fiber is that by selection of appropriate optical pulseparameters, and particularly pulse width in combination with the gaugelength of the interferometer with respect to the reflector spacing, thena multi-resolution distributed acoustic measurement can be obtainedsimultaneously. FIGS. 20 and 21 illustrate the arrangement in moredetail.

In FIG. 20 , assume we have a fiber with reflectors, which may, forexample, be the gratings described in the above embodiments, spacedevery 10 m. These are shown as reflectors 202, 204, and 206 on FIG. 20 .The reflectors are regularly spaced in at least one portion of the fiberwhere it is desired to undertake dual-resolution sensing. Of course, insome embodiments this may be along the whole length of the fiber. Inother embodiments, different sections of fiber may have reflectorsspaced at respective different spacings, such that different spatialresolutions are obtained from the respective different sections.

Now, given such a fiber, if we control the DAS system to produce opticalpulses to be sent into the fiber such that the pulse width is less thanthe reflector spacing, but where the gauge length of the DAS system(i.e. the path length difference 109 between the arms 105 and 107 of theDAS sensing interferometer 713) is greater than the reflector spacingthen as a pulse travels along the fiber there will be an instance duringwhich the respective reflected light from the pulse in the arms 105 and107 of the interferometer is from consecutive reflectors, in which caseduring this time a sensing resolution equal to the reflector spacing isobtained. There will next then be an instance where there is onlyreflected light in one of the interferometer arms and not the other (dueto the delay therebetween), in which case no output signal is obtained,and then following this there will then be an instance when therespective reflected light in the interferometer arms 105 and 107 isfrom a first reflector, and not the next reflector but instead thereflector next to that along the fiber i.e. 2 reflector spacings along,in which case at that time the sensing resolution is twice the reflectorspacing. Hence, with such operation a dual spatial resolution isobtained, alternating between a first spatial resolution and a second,doubled, resolution. FIG. 20 illustrates this concept in more detail.

For the sake of convenience, in FIG. 20 , a pair of pulses are showntravelling down the fiber, the fiber being provided with reflectors 202,202, and 206 spaced 10 m apart. The pair of pulses described here, forthe sake of ease of description, correspond to an actual pulse 210transmitted along the fiber from the DAS, and, in the case of receivinginterferometer arrangement, a virtual delayed pulse 212, delayed by thegauge length of the interferometer. Of course, in reality the virtualdelayed pulse 212 never actual travels along the fiber, but is insteadgenerated as a delayed version of the reflections of the actual pulse210 in the arm 105 of the interferometer 713, as described above.However, for the sake of descriptive convenience to illustrate theoperation of the present embodiment, the virtual delayed pulse canequally be thought of as virtually travelling along behind the actualpulse along the fiber, separated therefrom by the gauge length, and inthe following this model is adopted. However, it should be noted that inreality the delayed pulse is only ever in existence in the form ofreflected light from along the fiber from the actual pulse when delayedin arm 105 of the interferometer, and hence pulse 212 travelling alongthe fiber is a virtual pulse provided for the sake of descriptiveconvenience only.

With the above in mind, in this example the pulses have respectivelengths of 7.5 m, and the pulse separation (corresponding to the gaugelength) i.e. from falling (or rising) edge to falling (or rising) edgeis 15 m; hence there is a 7.5 m gap between the falling edge of theleading pulse and the rising edge of the following virtual pulse. Thesetimings between the pulses and relating to their lengths are maintainedas the pulses travel along the fiber.

At time A the leading actual pulse 210 is positioned so that it is stillpassing over reflector 204, such that some of the pulse is reflectedback along the fiber as the pulse passes over the reflector. Incontrast, the trailing virtual pulse 212 is just incident on reflector202, and hence is just about to start reflecting some of its light backalong the fiber (in reality of course, as noted above the lightconsidered to be reflected from the virtual pulse is actually earlierreflected light from the actual pulse, subject to the gauge length delayin the interferometer). Thus, between time A and B, a small portion ofactual pulse 210 is reflected from reflector 204 whilst a small portionof virtual pulse 212 is reflected from reflector 202. This reflectedlight from both pulses then travels back along the fiber where it canthen be processed in the DAS, for example by being interfered togetherin the DAS interferometer, so as to allow a DAS signal with resolutionequal to the distance between the reflectors 202 and 204 i.e. onereflector spacing, in this case 10 m to be found. Hence, between times Aand B DAS output with spatial resolution of 10 m i.e. one reflectorspacing is obtained.

Now consider the period from B to C. At time B leading actual pulse 210is no longer over reflector 204, and instead is between reflector 204and reflector 206. Hence, there is no reflection from this pulse.Trailing virtual pulse 212 is still “passing” over reflector 202 atpoint B and hence some light is reflected therefrom, but because thereis no light from actual pulse 210 to interfere with in the DAS, nosignal is produce at this time. This situation continues until time C,at which point leading actual pulse 210 then starts to pass over thenext reflector 206. At this time, trailing virtual pulse 212 is still“passing” over reflector 202, located two reflector spacings away fromreflector 206. Light is therefore reflected back along the fiber fromboth pulses, but this time from reflectors that are twice the distanceaway from each other than previously. Hence light from two reflectionpoints is available to be interfered in the DAS interferometer (orotherwise processed in the DAS) to obtain an output signal, but thistime because the distance between the sensing points is doubled, thespatial resolution of the DAS output signal is also doubled (or halved,depending on terminology) to twice the reflection point spacing, or 20 min this example.

This 20 m sensing resolution is then obtained from time C to time D,during which the leading actual pulse 210 passes over reflector 206, andthe trailing virtual pulse 212 passes over reflector 202. During time Cto D, therefore, a sensing resolution output of 20 m, or twice thereflector spacing, is obtained. At time D the trailing virtual pulse 212finishes passing over reflector 202, and hence from time D no signal isobtained, until trailing virtual pulse 212 is incident on the nextreflector 204 (not shown). At this point in time, the leading actualpulse 210 would still be passing over reflector 206, and hence the samesituation as shown at time A would pertain, except for the next pair ofreflectors along the fiber i.e. 204 and 206, rather than 202 and 204.The process therefore repeats, for each successive pair of reflectorsalong the fiber.

With the above therefore, what is obtained is a dual-resolution DASwhere the sensing resolution alternates between one reflector spacingand two reflector spacings as the actual pulse travels along the sensingfiber. This is an important result, because DAS systems suffer from whatis referred to as antenna effect, in that if the incident acousticwavelength, travelling along the fiber axis, is equal to the gaugelength then no signal is obtained; the fiber experiences an equal amountof tension and compression over the gauge length and so no meaningfulsignal is measured. However, with the auto dual resolution arrangementprovided by the use of regularly spaced reflectors and careful choice ofpulse width and gauge length in dependence thereon, antenna effect canbe negated at least at the longer resolution, as measurements at thesmaller resolution will also be being made automatically.

FIG. 21 illustrates the dual sensitivity arrangement again, togetherwith some simulated results. The principle is shown at FIG. 21A, wherean optical pulse and its delayed-over-L0 virtual echo travel along thefiber they cover consequently one or two zones between reflectorsseparated by the distance L˜⅔L0. In other words the sensitivity base ofthe DAS is changing along the fiber from L to 2L and back again, as thepulses travel therealong. This option was demonstrated by modelling forL0=15 m, with 75 ns pulse (˜7.5 m pulsewidth) and 1 m sampling fordifferent acoustic wavelengths. If the wavelength (Λ) is 60 m (so it issignificantly longer than L and L0, see on FIG. 21 B) then the DASoutput pattern (FIG. 21C) follows it, but some space zones have twicethe amplitude, see also a spectrum shown on FIG. 21D.

Further results for where the wavelength is equal to twice the reflectordistance L i.e. Λ=2L=20 m are presented on FIG. 21 E-G. In this casezones which correspond to 2L sensitivity base show no signal, due to theantenna effect mentioned above. This emphasises the advantage of theconcept: for long acoustic wavelengths i.e. where Λ>>2L then the 2L longsensitivity base can be used to improve SNR, but for short wavelengthsi.e. where Λ˜2L then the short base length L demonstrates better SNR.

The last pictures (FIG. 21 H-J) demonstrate a DAS output for where thewavelength is between the dual sensitivities i.e. between 10 m and 20 m.Generally, therefore, where L<Λ<2L. Here, signal to noise ratio (SNR) isstill moderate but special processing is necessary to transform theoutput pattern into a shape corresponding to the input (compare FIG. 21. H and I). One algorithm for such transformation is presented on FIG.23 . Here, a 2D vector of channels A is split into zones eachcorresponding to L and 2L (to give B and C), then each is filtered(including deconvolution if necessary) separately to produce Bf and Cfcorrespondently. A final result can then be produced by combining Bf andCf to give Af=Bf+Cf containing full flat spectrum with optimum SNR.

A further embodiment will now be described with respect to A to C ofFIG. 22 . Here, A and C of FIG. 22 show again the embodiments describedabove: an optical pulse (and its delayed-over-L0 virtual echo shown as alight grey rectangle) travelling along reflectors separated by thedistance L=L0. The pulsewidth in this case should be slightly less thanthe reflector separation, say 10 ns (˜1.0 m) for L=1.5 m.

Now consider an alteration of this setup where the gauge length (L0) isnow chosen to be a multiple larger than 1 of the reflector separation,see B of FIG. 22 , where L0=3L. In this case only the illuminatedreflectors i.e. those which a pulse is presently passing over produce areflected signal and so affect the output signal, and hence acousticantenna length can then be chosen from a range of lengths correspondingto different multiples of the distance between the reflectors tooptimise the output. That is, the gauge length can be chosen to selecthow many reflectors are between the pulse pairs, and hence the spatialresolution of the DAS may be controlled using a fibre with a fixed setof reflection points.

This option was demonstrated by modelling for a L0=4.5 m DAS, with a 10ns pulse and 1 m sampling for separation between reflectors of L=1.5 mand acoustic wavelengths Λ=7 m. A better SNR was found in this case thanfor cases where the pulse separation (from falling edge to falling edge)is equal to the spacing between the reflectors, for the cases where thespacing is small (e.g. ˜1.5 m) and larger (e.g. ˜9 m)-please compare Eof FIG. 22 with D of FIG. 22 or F of FIG. 22 , and it is seen that aclearer image is obtained.

With respect to selecting the spatial resolution, as mentioned above,this is performed by selecting the pulse width and the virtual pulseseparation (gauge length) characteristics, such that the desired numberof reflectors are encompassed by the sum of the pulse width and thepulse separation to give the resolution required. Thus, for example,from B of FIG. 22 it can be seen that by increasing the virtual pulseseparation (or gauge length) L0 so as to encompass a greater or lessernumber of reflectors, then a greater or lesser spatial resolution can beobtained. This further embodiment therefore gives a very convenient wayof changing the spatial resolution of the DAS system without requiringadditional hardware, and to allow fast, pulse to pulse changes ofresolution.

Various modifications and additions will now be described with respectto FIGS. 24 to 34 , in order to provide further embodiments of theinvention.

FIG. 24 illustrates one modification that may be made to any of theabove described embodiments. Here it is shown that the reflectors 1320need not be provided all the way along the fiber 1310, but instead canbe provided only in specific sections, with further sections of fiberthen in between in which no reflectors are provided. One or pluralsections of fibers may be provided each having multiple reflectorsprovided therein distributed therealong, These sections may then beinterspersed between conventional lengths of fibers having no reflectorstherein. The advantage of such an arrangement is that the range of theoptical fiber distributed sensing system can be increased, by onlyproviding the reflectors in those portions of the fiber that are locatedwhere sensing is actually desired. The lengths of fibers in betweenthose locations which are not provided with reflectors then effectivelybecome relatively lower-loss transmission portions for transporting theoptical pulses between the sensing portions provided with thereflectors. In effect, the optical fiber can be characterised as havingpulse transmission portions where no reflectors are provided, in betweenone or more sensing portions in which the reflectors are distributedtherealong.

In addition, with such an arrangement, it is possible to makedistributed acoustic measurements by measuring the backscatter betweenthe grating regions, using the iDAS. Such arrangements can beimplemented using a fast switch and attenuators along the detection pathas well as multiple interferometers in the iDAS for segments withdifferent spatial resolutions.

FIG. 25 illustrates a further variant of the above. Here three sensingportions 2510, 2520, and 2530 of fiber are provided, each provided withplural reflectors 1320. The sensing portions of fiber are dispersed atdifferent longitudinal positions along the whole fiber, and areconnected by transmission portions of fiber within which no reflectorsare provided, and hence which are relatively low loss for carrying theoptical pulses from sensing portion to sensing portion. In thearrangement of FIG. 25 , however, each sensing portion 2510, 2520, and2530 has reflectors that reflect different, substantiallynon-overlapping, wavelengths of light. That is, the reflectors in thefirst sensing portion 2510 reflect light around a μm, those of thesecond sensing portion 2520 reflect light around b μm, and those of thethird sensing portion 2530 reflect light around c μm. At wavelengthsthat the reflectors don't reflect the incident light is transmitted bythe reflectors with substantially no additional loss.

With such an arrangement the optical fiber distributed sensor system isable to provide spatial selectivity in terms of which set of reflectorsat which spatial location it wants to receive reflections from (andthereby enable sensing at that location), by varying the wavelengths ofthe transmitted pulses to match the reflector wavelengths of the set ofreflectors that are to be selected. Hence, varying the wavelengthsprovides the spatial selectivity of where the sensing system will sense,specifically which set of reflectors will provide reflections from whichsensing can then be undertaken.

Additionally, because the non-selected reflectors do not reflectsubstantially at the wavelengths of the pulses being transmitted alongthe fiber for the selected set of reflectors, losses from unwantedreflections are kept to a minimum, and the sensor range is increased.

FIG. 26 illustrates a further modification that may be made to providefurther embodiments of the invention. Here, the fiber 1310 is againconnected to an optical fiber distributed sensor system (not shown), andis provided in at least one sensing portion thereof (or alternatively,all along its length) with reflector portions the reflectivity of whichare different along the length of the fiber. In particular, in oneembodiment shown in FIG. 26 the reflectivity of the reflector portionsincreases along the length of the fiber with distance from the opticalpulse source in the optical fiber sensing system, such that reflectorportions further away from the source have greater reflectivity thanthose nearer the source.

The reflectivity in some embodiments increases deterministically inaccordance with a mathematical function of the distance along the fiber.For example, the mathematical function may be a monotonic functionrelating distance along the fiber to reflectivity.

One of the primary motivations for altering the reflectivity of thereflectors along the fiber is to account for crosstalk between thereflectors. Crosstalk results from unwanted light, which has undergonereflections off multiple reflectors, returning coincidently with thesignal of interest, which undergoes one reflection only. Whereas thewanted optical power strength will be proportional to R (thereflectivity of a single reflector), the crosstalk signal (if we ignoreoptical losses and assume equal reflectivity for all reflectors) will beapproximately proportional to N×R³, where N is the number of opticalpaths that allow the crosstalk light to arrive at the detectorcoincidently with the signal light.

The crosstalk can be minimised by reducing both N and R; however theuseful optical signal is maximised by increasing R and the spatialresolution is optimised by increasing the number of reflectors, andhence N. Thus a compromise between crosstalk, spatial resolution andsignal to noise ratio (which is governed by the optical signal level)must be found for a particular target application through theappropriate choice of N and R.

Note, N, and hence the amount of crosstalk experienced by the signal,increases along the fibre length. For example, there is no crosstalk forthe first pair of reflectors as there is no valid crosstalk optical pathwhich allows the crosstalk light to arrive coincidently with the signallight. Similarly, the contribution of crosstalk from acoustic signalgenerated towards the beginning of the fibre is larger than thecontribution of the same signal level towards the end of the fibre. Thisis because there are many more valid crosstalk optical paths whichencompass reflectors towards the beginning of the fibre than towards theend. For example, a reflection off the last reflector in the fibre hasno valid path where it can contribute to crosstalk whereas a reflectionoff the first reflector can contribute to crosstalk by reflecting lightfrom any other reflectors.

This means that if the acoustic signal level is constant along thefibre, and if the reflectivity of the reflectors is also constant, theinfluence of the crosstalk increases along the fibre length, and theacoustic signal impinging on the near end of the fibre contributes moreto the crosstalk than that impinging on the far end of the fibre.

In order to address this issue an elegant approach to optimiseperformance in response to this property is to vary the reflectivity ofthe reflectors along the fibre length. In this case, the nearerreflectors (which contribute more to crosstalk) are chosen to have alower reflectivity than the far reflectors. In this way is it possibleto equalise, or otherwise tune as wished, the crosstalk response of thefibre. This type of reflectivity profile is also beneficial in that thereflectivity can be tuned to also compensate for the optical losses inthe fibre (and through any connectors/splices or other losses along theoptical path) and so equalise, or otherwise tune, the signal to noiseperformance, as well as tune crosstalk, along the fibre.

In addition, the crosstalk contribution from regions along the opticalpath with a large acoustic signal of low importance (for example loudsurface noise in an oil well installation) can be negated by choosing alow reflectivity reflectors (or no reflectors) along that section.

In some applications, the signal of interest is towards the far end ofthe installation (for example the perforated section at the bottom of anoil well). In this case, the crosstalk contribution from the near end ofthe installation (the top of the oil well, which may be very noisy) canbe minimised by deploying the fibre in a “U” arrangement where thereflectors may be positioned in the far end of the leading fibre and maybe then continued to the top of the return fiber. In this case, thelaser light is launched down the fibre leg with no reflectors, such thatthe first reflector encountered is at the bottom of the well. Thisensures a good crosstalk behaviour as the region of interest at thebottom of the well is positioned first in the optical path, and soencounters minimal crosstalk. Also the loud section, at the top of thewell, is positioned at the end of the optical fibre and so does notcontribute crosstalk to the majority of the optical path, including theparticular region of interest.

FIG. 27 illustrates one way of forming the reflector portions in thecladding of the fiber, rather than in the core. Here, instead of thegratings being formed in the core of the fiber itself, they are insteadformed in the cladding layer, and coupled into the core via waveguides2710. In use light propagating along the core couples into thewaveguides 2710 and is fed to the gratings 1320, from where it is thenreflected back along the fiber.

FIG. 29 illustrates a further embodiment, related to adapting the pulserepetition rate of the DAS. As described previously in embodiments ofthe invention the DAS operates by sending an optical pulse along thefiber, and then measuring reflections from reflector portions positionedalong the fiber, as described above. We describe above how it ispossible to only provide reflectors in a single portion of the fiber,located where sensing is desired. FIG. 29 shows in simplified form suchan arrangement, where a single set of reflectors is provided at a singlesensing portion of the fiber, with the remainder of the fiber betweenthe sensing portion and the DAS being substantially free of sensors.

With such an arrangement, usually if it was desired to sense along thewhole length of the fiber then one pulse at a time would be transmittedon to the fiber, with the time between pulses equal at least to thespeed of the pulse along the fiber, plus the return time for backscatterfrom along the length of the fiber. Of course, given the speed of lightin the fiber this still allows for very high pulse repetition rates, andhence high sampling frequencies, typically as high as 100 kHz.

However, if it is desired to sense only along a smaller sensing portionof the fiber, such as the sensing portion provided with reflectors, thenthe pulse transit time that is important is the transit time for thepulse to transit the sensing portion only, plus the transit time forbackscatter from along the length of the sensing portion. If the pulsetransit time is τr, then allowing time for the backscatter the minimumpulse spacing is 2τr, plus typically some sort of small guard time g inbetween pulses, which may be say 10% of τr. The pulse repetition ratecan then be increased to 1/(2 τr+g), which, depending on the relativelength of the sensing portion compared to the whole fiber length, willbe significantly higher than the pulse repetition rate required toprovide whole length of fiber sensing. As a consequence, the samplingfrequency of the DAS can be increased, so as to allow the DAS to detecthigher frequencies.

Generally, the pulse repetition rate of the DAS can be increased by afactor equal to the ratio of the transmission portion of the fiber tothe sensing portion. For example, therefore, if the sensing portion isprovided only along one quarter of the length of the fiber then thepulse repetition rate may be increased by a reciprocal amount i.e. by afactor of four.

FIG. 30 shows a further arrangement where two DAS systems aremultiplexed on to a single fiber, and the fiber is provided withreflectors in two distinguishable sensing portions, being a firstportion where the spacing between the reflectors is greater than thespacing between reflectors in a second portion. The reflectors in thefirst portion are arranged to reflect a first wavelength a μm, and thereflectors in the second portion are arranged to reflect a secondwavelength, b μm. As shown, in this example the reflectors in the firstportion are spaced by 10 m, and the reflectors in the second portion arespaced more closely together to give a higher spatial sensing resolutionof 1 m.

Two DAS systems, DAS1 and DAS2 are provided both multiplexed onto thesingle sensing fiber. DAS1 operates at the first wavelength a μm,whereas DAS2 operates at the second wavelength b μm. Providing two DASsystems multiplexed on to the same fiber allows for simultaneousmulti-frequency operation, which in this case provides for simultaneousmulti-spatial resolution operation, due to the different reflectorspacings in the two sensing portions. Hence, with such an arrangementmultiple sensing resolutions can be obtained simultaneously fromdifferent parts of the same fiber.

In a variant of the above, instead of the different wavelengthreflectors being provided in different portions of the fiber, they areinstead provided along the length of the fiber in overlapping portions,but at the same respective spacings. Because the reflectors are arrangedto reflect at different wavelengths, however, being the first and secondwavelengths of the DAS1 and DAS2 systems respectively, there is nointerference of the respective operations of the two DAS systems, andmulti-spatial resolution sensing at the two resolutions is then obtainedalong the length of the same fiber.

FIG. 31 shows a multi-mode fiber embodiment, where the reflectorgratings are provided within the multi-mode core, as shown. As is knownin the art, multi-mode fiber cores are much wider than single mode fibercores, but as the DAS systems are internally single mode systems it issufficient to locate reflector gratings in the center of the core toreflect the lowest order mode, which is the mode that will typically becoupled into the DAS. FIG. 28 illustrates an alternative version, whereinstead of gratings, weakly reflecting right-angled reflectionstructures are formed in the core instead. The operation of such is thesame as if gratings were being used, but with the difference that thereflection structures are truly broadband, and will reflect some of theincident light of any wavelength propagating in the fiber.

FIGS. 32 and 33 show multi-core embodiments of the invention. Usingmulti-core fibers opens up the concept of having “forward” channels forthe forward pulse launched from the DAS or DTS system and “return”channels into which the forward light can be reflected for return to theDAS or DTS system. The advantage of having separate forward and returnchannels is that the return channel will have no backscatter thereonfrom the forward pulse, and hence a higher signal to noise ratio can beobtained.

FIG. 32 illustrates the basic concept with a multi-core fiber. Here,core 2 is the forward channel onto which the optical pulses from the DASor DTS system are launched. Co-operatively angled reflectors 3210 and3220 are provided, one for each of core 1 and core 2, and each angled at45 degrees to their respective cores, with a 90 degree angle to eachother. Such an arrangement means that an optical pulse travelling alongthe forward channel provided by core 2 is reflected through 90 degreesfrom reflector 3210 into core 1, and the reflected through a further 90degrees by reflector 3220 so as to travel in the opposite returndirection back towards the DAS or DTS system along core 1. As notedabove, because core 1 does not carry the forward pulse, there are nounwanted reflections or backscatter on core 1, the only light carriedback to the DAS system is the reflected light on core 1.

FIG. 33 extends the concept further, by providing two return cores, inthe form of core 1 and core 3. Providing two (or more) return coresopens up the prospect of multi-spatial sensing resolution along thefiber, by providing different reflector spacings on each of the returncores. In the example shown, corner angle reflectors as described aboveare provided between the forward core 2 and return core 1 with a spacingof r1, and between forward core 2 and return core 3 with a spacing ofr2, wherein r2>r1. With such an arrangement, the combination of forwardcore 2 and return core 1 provide for sensing with a spatial resolutionrelated to r1, and the combination of forward core 2 and return core 3provide for sensing with a different, longer, spatial resolution r2.

In further variants of this embodiment, further return cores may beprovided, having reflectors at even larger or even smaller spacings, toprovide for even more spatial resolutions. Moreover, in some embodimentsthere is no limitation to having just a single forward core, anddepending on the number of cores there may be more than one forwardcore, each surrounded by a plurality of return cores, with differentspaced reflectors from return core to core. Thus, many different spatialresolutions may be measured simultaneously using multi-core fibers.

FIGS. 34-1 and 33-2 show a further embodiment of the invention. In FIG.34-1 , consecutive reflector portions 1320 alternate in reflectingdifferent wavelengths a μm and b μm along the fibre, with particularrespective reflection bandwidths, which overlap, as shown. The laserwavelength is selected to be within the overlapping wavelength region,such that it is reflected by all the reflector portions 1320, andspecifically is a wavelength distance εz microns away from the centerreflecting frequency of the reflector that reflects at a microns, and isa wavelength distance ε(z+1) away from the center reflecting frequencyof the reflector that reflects at b microns. With such an arrangement itis possible to use the reflective bands to measure the change in theintensity of the reflectors directly, to thereby get a measure of thestatic strain along the fibre regions.

FIG. 34-2 builds upon the arrangement of FIG. 34-1 , by providing thatthe reflectors 1320 have three respective reflecting wavelengths amicrons, c microns, and b microns, in order, repeating along the fiber.The laser wavelength is at c microns, again separated from a microns bya wavelength distance εz microns away from the center reflectingfrequency of the reflector that reflects at a microns, and again awavelength distance ε(z+1) away from the center reflecting frequency ofthe reflector that reflects at b microns. Again, with such anarrangement it is possible to use the reflective bands to measure thechange in the intensity of the reflectors directly, to thereby get ameasure of the static strain along the fibre regions.

Turning to a consideration of the reflector spacing, the spacing betweenthe reflectors need not be regular in embodiments of the invention, anda variable spacing is possible. The spacing may vary by as much as 10 to20%, provided that there is pulse overlap between reflections from theactual pulse and virtual pulse in the interferometer.

In addition, and as mentioned previously, the grating spacing can changealong the length of the fiber, for example the spacing may be largerbetween gratings the further along the fiber from the DAS. Moreover, thereflectivity of the gratings may also increase the further along thefiber from the DAS. One particularly preferred spacing of the reflectorsis to have the spacing at half the gauge length of the DAS, being thelength difference between the different arms of the interferometer inthe DAS (e.g. so for 10 m gauge length we have a 5 m reflector spacing).

With respect to the specifications of the gratings forming thereflectors in embodiments of the invention, the gratings may be writteninto the fiber as the fiber is produced, as is known in the art orwritten in the fiber after the fiber has been produced, as is also knownin the art. The reflective strength of each reflector may be between −30and −60 dB, and more preferably −40 to −50 dB and even more preferablyaround −45 dB. The total reflectors reflectivity all-together may bebetween −10 and −30 dB.

FIGS. 35 and 36 are two respective graphs, which further particularisethe specifications of the reflectors, in terms of their number, theirreflectivity, and the reflection bandwidth. Specifically, reviewing FIG.35 , it can be seen that the number of sensing points i.e. the number ofreflectors is inversely related to the reflectivity of the reflectors,for a given acceptable level of cross-talk, in that more reflectorpoints can be provided when the reflectivity of the reflector points islower. Moreover, for a given number of reflectors, the reflectivity ofthose reflectors is also related to the desired or allowable cross-talk,in that if a higher cross-talk is acceptable, then a higher reflectivitycan be used. Thus, when specifying a fiber for a particular application,the number of sensing points can first be specified, based on thesensing length of the fiber (i.e. the length over which sensing needs totake place), and the desired spatial resolution which in turn definesthe spacing between the reflectors. Then, having determined the numberof reflectors needed (in dependence on the desired length of fiber overwhich sensing needs to take place and the desired spatial resolutionover that length), an acceptable level of cross-talk can then bespecified, which in turn then allows the reflectivity of the reflectorsto be determined, in accordance with the graphed functions.

Another consideration is the reflection wavelength width of thereflectors, concerning over what wavelength the reflectors reflect. FIG.36 illustrates that the wavelength width is temperature dependent, inthat temperature changes at the fibre cause the reflector gratingreflection peak wavelength to change. In addition, a broad reflectionbandwidth is also desirable in order to cope with changes in strain onthe fibre in addition to changes in temperature. This is because strainas well as temperature changes the centre wavelength of a gratingreflector.

As will be seen from FIG. 36 , the desired wavelength width is operatingtemperature dependent, and also dependent on the condition of the fiber,for example whether it is attached to any structure or the like.Specifically, the desired wavelength width is proportionally related tothe operating temperature range, in that the greater the temperaturerange, the greater the wavelength width required. In practice, arelatively broadband wavelength width around the laser wavelength isdesirable, to allow for temperature changes irrespective of whether suchchanges take place. A linewidth of at least +/−2 nm, or more preferablyat least +/−3 nm, or more preferably at least +/−4 nm, or even morepreferably at least +/−5 nm is therefore desirable.

One parameter that can be used as a convenient design parameter for thefiber is the preferred “NR” (where NR is the number of markers (N)multiplied by average reflectivity (R) of the markers along the fiber)In order to obtain good performance, given the general aims of reducingcrosstalk and providing the desired spatial resolution, a maximum NR of10% is preferred.

FIG. 37 shows a further embodiment that addresses the issue of crosstalkby making use of multiple fiber channels. These can be discreteindividual fibers running together, for example in parallel, or could bea single multi-core fiber, or combinations of the two. Whatever theconfiguration of the fiber, the result is that multiple fiber channelsare provided, which are multiplexed together at one end and connected toa DAS, the DAS being as described previously. The individual fiberchannels are provided with respective regions therein where reflectorsare provided, with the rest of the individual fiber channel being freeof reflectors, to reduce cross talk and other losses. The longitudinalpositioning of the respective regions from fiber to fiber along thelength of the parallel fibers is such that regions are essentiallylongitudinally contiguous along, and either do not longitudinallyoverlap, or only very partially overlap.

The result of the above arrangement is that sensing can be provided asif a single fiber had reflectors all the way therealong, but with muchreduced crosstalk than such a case. This is because per parallel fiberchannel there are in fact fewer reflectors, than the case of a singlefiber, by a factor related to the number of individual fiber channels.For example, where there are 4 fiber channels, then the number ofreflectors which would otherwise be required to provide sensing at thedesired spatial resolution all along a single fiber length can bedivided into 4 groups, one group per fiber, located longitudinally alongthe fibers in respective contiguous groups, as shown. This means thatthe number of reflector points per individual fiber channel is alsoreduced to a quarter of the number, which from FIG. 35 above, means thatfor a given desired level of crosstalk a higher reflectivity can beused, or conversely, for the same reflectivity a lower level ofcrosstalk is obtained.

In order to implement the above an arrangement such as that shown inFIG. 37 (b) or (c) should be used. Unfortunately, simple lightseparation with a 1×N coupler as shown in (a) of FIG. 37 cannot be used,because coupler losses mean that the reflected light reaching the DASsystem remains the same. However, the coupler losses can be overcome bymaking use of bidirectional application on each fiber length, as shownin (b) of FIG. 37 , or by fast optical switching between the individualparallel fiber lengths, as shown in (c) of FIG. 37 . Fast integratedoptical switches with 2 ns switching times are available, for example,from companies such as PhotonIC Corp, of Culver City, CA, USA.

The above described embodiments have focused on the application of theinvention to an optical fiber distributed acoustic sensor system.However, the optical fiber described herein can also be used withoptical fiber distributed temperature sensor systems, such as theSilixa® Ultima™ DTS system, described at silixa.com. For example, theprior art Silixa® Ultima™ DTS system can measure 0.3 nm for 10 m gaugelength, and hence for 10 cm gauge length the resolution would be 30 nm.The fibre temperature coefficient is about 10⁻⁵/° K/m. For 10 cm 1 um/°K or 1 nm/° mK. However, using a sensing fiber with wideband weakreflectors therein as described above we can improve by ×10 andtherefore for 10 cm we should be able to measure 3° mK at 10 kHz.Averaging to 10 Hz the performance should then approach 0.1° mK.

In summary, therefore, embodiments of the present invention provide animproved optical fiber distributed sensor system that makes use of aspecially designed optical fiber to improve overall sensitivity of thesystem, in some embodiments by a factor in excess of 10. This isachieved by inserting into the fiber weak broadband reflectorsperiodically along the fiber. The reflectors reflect only a smallproportion of the light from the DAS incident thereon back along thefiber, typically in the region of 0.001% to 0.1%, but preferably around0.01% reflectivity per reflector. In addition, to allow for temperatecompensation to ensure that the same reflectivity is obtained if thetemperature changes, the reflection bandwidth is relatively broadbandi.e. in the region of +/−3 nm to +/−5 nm from the nominal laserwavelength. In some embodiments the reflectors are formed from a seriesof fiber Bragg gratings, each with a different center reflectingfrequency, the reflecting frequencies and bandwidths of the gratingsbeing selected to provide the broadband reflection. In other embodimentsa chirped grating may also be used to provide the same effect. Inpreferred embodiments, the reflectors are spaced at half the gaugelength i.e. the desired spatial resolution of the optical fiber sensorsystem. The optical fiber distributed sensor system may be any of anacoustic sensor system, vibrational sensor system, temperature sensorsystem, or any other sensed parameter that perturbs the path length ofthe optical fiber.

Various further modifications, whether by way of addition, deletion, orsubstitution may be made to above mentioned embodiments to providefurther embodiments, any and all of which are intended to be encompassedby the appended claims.

There follows a list of numbered features defining some embodiments ofthe invention. Where a numbered feature refers to one or more othernumbered features then those features may be considered together incombination.

1. An optical fiber distributed sensor system, comprising:

-   -   an optical source arranged in use to produce optical signal        pulses;    -   an optical fiber deployable in use in an environment to be        sensed and arranged in use to receive the optical signal pulses;        and    -   sensing apparatus arranged in use to detect light from the        optical signal pulses reflected back along the optical fiber and        to determine any one or more of an acoustic, vibration,        temperature or other parameter that perturbs the path length of        the optical fiber in dependence on the reflected light;

the system being characterized in that the optical fiber comprises aplurality of reflector portions distributed along its length in at leasta first sensing region thereof, the reflectivity of the reflectorportions being inversely dependent on:

-   -   i) the number of reflector portions in the at least first        sensing region; and    -   ii) a selected amount of crosstalk between the reflector        portions in the at least first sensing region.

2. A system according to feature 1, wherein spacings between thereflector portions are set in dependence on timing characteristics ofthe optical signal pulses.

3. A system according to feature 1, wherein timing characteristics ofthe optical signal pulses are set in dependence on spacings between thereflector portions.

4. A system according to features 2 or 3, wherein the timingcharacteristics include one or more of the pulse width and the gaugelength.

5. A system according to any of features 1 to 4, wherein the gaugelength of the sensor system is dependent on i) the spacings between thereflector portions; and/or ii) the timing characteristics of the opticalsignal pulses.

6. A system according to any of the preceding features, wherein thespacings between the reflector portions are any of a fraction, equal toor a multiple of the gauge length of the sensor system.

7. A system according to any of the preceding features, wherein thespacings between the reflector portions and/or the timingcharacteristics of the optical signal pulses are selected such that adual resolution output signal is obtained, that alternates between afirst spatial resolution and a second spatial resolution as the opticalsignal pulses travel along the fiber.

8. A system according to feature 7, whereby the spacings between thereflector portions are less than the sum of the pulse width and thepulse separation of the optical signal pulses.

9. A system, according to feature 8, wherein the second spatialresolution is substantially twice that of the first spatial resolution.

10. A system according to any of the preceding features, wherein thereflector portions have a small reflectance over a large bandwidth.

11. A system according to feature 10, wherein the reflector reflectanceis less than 1%, and preferably less than 0.1%.

12. A system according to feature 11, wherein the reflector reflectanceis less than 0.1% and more than 0.001%, preferably in the range 0.05% to0.005%, and more preferably around 0.01%.

13. A system according to any of the preceding features, wherein atleast one of the reflector portions comprises a plurality of fiber Bragggratings.

14. A system according to feature 13, wherein the plurality of fiberBragg gratings are arranged in an array along a reflector portion,individual gratings in the array having non-identical but overlappingreflection bandwidths to the other gratings in the array whereby toprovide a broadband reflector.

15. A system according to feature 14, wherein the array length is in theregion of 30 to 60 mm, and preferably 40 to 50 mm, with the gratingsarranged substantially equally over the array length.

16. A system according to feature 15, wherein there are at least three,and more preferably 4 or 5 gratings in the array.

17. A system according to any of features 1 to 12, wherein a reflectorportion comprises a chirped grating.

18. A system according to any of the preceding features, wherein thereflectance bandwidth of a reflector portion is selected such that thereflector portions reflect the optical signal pulses over an expectedrange of operating temperatures of the system.

19. A system according to any of the preceding features, wherein thereflectance bandwidth is at least +/−2 nm around the wavelength of theoptical source, and more preferably at least +/−3 nm, and even morepreferably at least +/−5 nm

20. A system according to any of the preceding features, wherein thereflector portions are regularly distributed along the length of thefiber in at least the first sensing region.

21. A system according to any of features 4, 5, or 6, wherein the gaugelength of the sensor system is a minimum length of fiber over which anacoustic signal incident on the fiber can be resolved.

22. An optical fiber distributed sensing system, comprising:

-   -   an optical fiber deployable in an environment to be sensed, the        optical fiber having reflector portions regularly distributed in        at least a first region thereof and having a first spacing        therebetween;    -   an optical signal source arranged in use to input optical pulses        into the optical fiber; and    -   sensing apparatus arranged in use to detect light from the        optical pulses reflected back along the optical fiber and to        determine any one or more of an acoustic, vibration, temperature        or other parameter that perturbs the path length of the optical        fiber in dependence on the reflected light;    -   wherein the optical signal source is controlled to produce        optical pulses of a first pulse width selected in dependence on        at least the first spacing of the reflector portions in such a        manner that the sensing apparatus determines acoustic signals of        a first spatial resolution and a second spatial resolution        alternately.

23. A system according to feature 22, wherein the first pulse width isless than the first spacing.

24. A system according to features 22 or 23, wherein the first spacingis less than the gauge length of the system.

25. A distributed sensing system, comprising:

-   -   an optical fiber deployable in an environment to be sensed, the        optical fiber having reflector portions regularly distributed in        at least a first region thereof and having a first spacing        therebetween;    -   an optical signal source arranged in use to input optical pulses        into the optical fiber; and    -   sensing apparatus arranged in use to detect light from the        optical pulses reflected back along the optical fiber from the        reflector portions and to determine any one or more of an        acoustic, vibration, temperature or other parameter that        perturbs the path length of the optical fiber in dependence on        the reflected light;    -   wherein the optical signal source is controlled to adjust the        timing characteristics of the optical pulses in dependence on        the first spacing so as to select a desired spatial sensing        resolution.

26. A system according to feature 25, wherein the timing characteristicsare adjusted such that a gauge length of the DAS encompasses a pluralityof first spacings.

27. A system according to features 25 and 26, wherein the timingcharacteristics are adjustable to select one of a plurality of possiblespatial resolutions.

28. A system according to any of features 25 to 27, wherein the gaugelength is adjusted to encompass one or more first spacings to select adesired spatial sensing resolution.

29. A distributed sensing system, comprising:

-   -   an optical fiber deployable in an environment to be sensed, the        optical fiber having reflector portions regularly distributed in        at least a first region thereof and having a first spacing        therebetween;    -   an optical signal source arranged in use to input optical pulses        into the optical fiber; and

sensing apparatus arranged in use to detect light from the opticalpulses reflected back along the optical fiber from the reflectorportions and to determine any one or more of an acoustic, vibration,temperature or other parameter that perturbs the path length of theoptical fiber in dependence on the reflected light;

-   -   wherein a gauge length of the sensing apparatus is adjusted to        encompass one or more first spacings to select a desired spatial        sensing resolution.

30. A distributed sensing system according to any of the precedingfeatures, wherein the reflectivity of the reflector portions alters independence on the position of the reflector portion along the fiber.

31. A system according to feature 30, wherein the reflectivity of thereflector portions increases in dependence on any one or more of:

-   -   i) the distance along the fiber from the optical signal source;        and/or    -   ii) the optical loss along the fiber from the optical signal        source; and/or    -   iii) the optical losses of connectors and feedthroughs.

32. A system according to any of the preceding features, wherein thereflector portions comprise reflective gratings formed in the claddingof the fiber, respective waveguides being provided into the core of thefiber to couple light energy from the core to the reflective gratings.

33. A system according to any of the preceding features, wherein thereflector portions may each be respectively arranged to have areflectance bandwidth selected from a set of bandwidths to be reflected.

34. A system according to feature 33, wherein consecutive reflectorportions reflect different reflectance bandwidths to each other.

35. A system according to feature 34, wherein a first reflector portionreflects a first reflectance bandwidth centered around wavelength amicrons, and a second reflector portion located consecutively next tothe first reflector portion reflects a second reflectance bandwidthcentered around wavelength b microns, where a<b.

36. A system according to feature 35, wherein a third reflector portionlocated consecutively next to the second reflector portion reflects athird reflectance bandwidth centered around wavelength c microns.

37. A system according to feature 36, wherein b<c, or alternatively b>c.

38. A system according to any of the preceding features, wherein thefiber is a multi-mode fiber, and the reflector portions compriserespective gratings formed in the core of the multi-mode fiber atrespective positions so as to cause reflection of the same propagationmode.

39. A system according to feature 38, wherein the gratings are formedsubstantially centrally in the core of the multi-mode fiber whereby toreflect energy from the lowest order mode.

40. A system according to any of the preceding features, wherein theoptical fiber is a multi-core fiber, the reflector portions comprisingfirst reflectors arranged to reflect a portion of the input opticalpulses from a first one of the multiple cores onto which in use theoptical pulses are input to a second one of the multiple cores, and todirect the reflected portion back along the second one of the multiplecores towards the sensing apparatus, the sensing apparatus beingarranged in use to detect the reflected light from the second one of themultiple cores.

41. A system according to feature 40, and further comprising a third oneof the multiple cores, second reflectors being provided to reflect aportion of the input optical pulses from the first core to the thirdcore and to direct the reflected portion back along the third one of themultiple cores towards the sensing apparatus, the sensing apparatusbeing arranged in use to detect the reflected light from the third oneof the multiple cores.

42. A system according to feature 41, wherein the distance between thefirst reflectors coupling the first core to the second core is differentto the distance between the second reflectors coupling the first core tothe third core, whereby different spatial sensing resolutions areobtained from the multiple cores.

43. An optical fiber distributed sensor system, comprising:

-   -   a first optical source arranged in use to produce optical signal        pulses of a first wavelength;    -   a second optical source arranged in use to produce optical        signal pulses of a second wavelength;    -   an optical fiber deployable in use in an environment to be        sensed and arranged in use to receive the optical signal pulses;    -   a first sensing apparatus arranged in use to detect light from        the optical pulses of the first wavelength reflected back along        the optical fiber and to determine acoustic signals incident on        the optical fiber in dependence on the reflected light; and    -   a second sensing apparatus arranged in use to detect light from        the optical pulses of the second wavelength reflected back along        the optical fiber and to determine any one or more of an        acoustic, vibration, temperature or other parameter that        perturbs the path length of the optical fiber in dependence on        the reflected light;    -   wherein the optical fiber is provided with first reflector        portions arranged to reflect at least a portion of signals of        the first wavelength in a first section of the optical fiber,        and the with second reflector portions arranged to reflect at        least a portion of signals of the second wavelength in a second        section of the optical fiber.

44. A system according to feature 43, wherein the first reflectorportions are spaced apart from each other differently to the secondreflector portions, whereby to provide different spatial sensingresolutions in the first and second sections of the optical fiber.

45. An optical fiber distributed sensor system, comprising:

-   -   an optical source arranged in use to produce optical signal        pulses;    -   a plurality of optical fiber core lengths deployable in use in        an environment to be sensed    -   an optical coupler or optical switch arranged in use to receive        the optical signal pulses and to couple or switch them into the        plurality of optical fiber core lengths; and    -   sensing apparatus arranged in use to detect light from the        optical signal pulses reflected back along the optical fiber        core lengths via the optical coupler or switch and to determine        any one or more of an acoustic, vibration, temperature or other        parameter that perturbs the path length of the optical fiber in        dependence on the reflected light;

wherein the optical fiber core lengths comprise respective pluralitiesof reflector portions distributed along the lengths in at leastrespective sensing regions thereof.

46. An optical fiber distributed sensor system according to feature 45,wherein the respective sensing regions are offset one from another alongthe respective optical fiber core lengths.

47. An optical fiber distributed sensor system according to feature 46,wherein the respective sensing regions are longitudinally contiguousalong the respective optical fiber core lengths.

48. An optical fiber distributed sensor system according to any offeatures 45 to 47, wherein the plurality of optical fiber core lengthsare provided by:

-   -   i) respective single core fibers arranged to run in parallel; or    -   ii) a multi-core fiber.

49. An optical fiber distributed sensor system according to any of thepreceding features, wherein the system is an optical fiber distributedacoustic sensor system arranged to sense acoustic signals incident uponthe optical fiber.

50. An optical fiber distributed sensor system according to any of thepreceding features, wherein a product of the number of reflectorportions and the average reflectivity of the reflector portions is 0.1or less.

What is claimed is:
 1. An optical fiber distributed sensor system,comprising: an optical source arranged in use to produce optical signalpulses; an optical fiber deployable in use in an environment to besensed and arranged in use to receive the optical signal pulses; andsensing apparatus arranged in use to detect light from the opticalsignal pulses reflected back along the optical fiber and to determineany one or more of an acoustic, vibration, or temperature parameter thatperturbs the path length of the optical fiber in dependence on thereflected light; the system being characterized in that the opticalfiber comprises: at least one first sensing region having a plurality ofreflector portions distributed along its length; and at least one pulsetransmission portion being a section of optical fiber in which noreflector portions are provided, the at least one pulse transmissionportion being located between the optical source and the at least onefirst sensing region, and configured in use to transport the opticalsignal pulses from the optical source to the at least one first sensingregion having the reflector portions, wherein the at least one pulsetransmission portion has a length greater than a distance between thereflector portions of the at least one first sensing region, and thesystem is an optical fiber distributed acoustic sensor system arrangedto sense acoustic signals incident upon the optical fiber.
 2. A systemaccording to claim 1, wherein the sensing apparatus determines the anyone or more of an acoustic, vibration, or temperature parameter thatperturbs the path length of the optical fiber in dependence on thereflected light from the reflector portions for the at least one firstsensing region, and determines the any one or more of an acoustic,vibration, or temperature parameter that perturbs the path length of theoptical fiber in dependence on backscatter from the at least one pulsetransmission portion.
 3. A system according to claim 1, wherein aplurality of separate first sensing regions are provided each havingreflector portions formed therein, connected in series by transmissionportions of fiber where no reflector portions are formed.
 4. A systemaccording to claim 3, wherein the plurality of separate first sensingregions have respective sets of reflector portions that are arranged toreflect different wavelengths of light, wherein the optical fiberdistributed sensor system is able to provide spatial selectivity interms of which set of reflector portions at which spatial location itwants to receive reflections from and thereby enable sensing at thatlocation, by varying the wavelengths of the transmitted pulses to matchthe reflector wavelengths of the set of reflector portions that are tobe selected.
 5. A system according to claim 1, wherein the optical fiberis deployed in a U shape having an outward leg of optical fiber and areturn leg of optical fiber, where the reflector portions in the atleast one first sensing region are positioned at the far end of theoutward leg of the optical fibre and continue to the top of the returnleg of optical fiber.
 6. A system according to claim 5, wherein theoptical fiber U-shape is deployed into an oil well, wherein the firstreflector portion encountered in the at least one first sensing regionis at the bottom of the well.
 7. A method of operating an optical fiberdistributed sensor system, comprising: using an optical source arrangedin use to produce optical signal pulses; using an optical fiber deployedin an environment to be sensed to receive the optical signal pulses; andusing a sensing apparatus to detect light from the optical signal pulsesreflected back along the optical fiber and to determine any one or moreof an acoustic, vibration, or temperature parameter that perturbs thepath length of the optical fiber in dependence on the reflected light;the method being characterized in that the optical fiber comprises atleast one first sensing region having a plurality of reflector portionsdistributed along its length and at least one pulse transmission portionbeing a section of optical fiber in which no reflector portions areprovided, the pulse transmission portion being located between theoptical source and the at least one first sensing region, and configuredto transport the optical signal pulses from the optical source to the atleast one first sensing region having the reflector portions, whereinthe at least one pulse transmission portion has a length greater than adistance between the reflector portions of the at least one firstsensing region, and the system is an optical fiber distributed acousticsensor system arranged to sense acoustic signals incident upon theoptical fiber.
 8. A method according to claim 7, and further comprisingusing the sensing apparatus to determine the any one or more of anacoustic, vibration, or temperature signal that perturbs the path lengthof the optical fiber in dependence on the reflected light from thereflector portions for the at least one first sensing region, anddetermines the any one or more of an acoustic, vibration, or temperatureparameter that perturbs the path length of the optical fiber independence on backscatter from the at least one pulse transmissionportion.
 9. An optical fiber distributed sensor system, comprising: afirst optical source arranged in use to produce optical signal pulses ofa first wavelength; a second optical source arranged in use to produceoptical signal pulses of a second wavelength; an optical fiberdeployable in use in an environment to be sensed and arranged in use toreceive the optical signal pulses; a first sensing apparatus arranged inuse to detect light from the optical pulses of the first wavelengthreflected back along the optical fiber and to determine acoustic signalsincident on the optical fiber in dependence on the reflected light; anda second sensing apparatus arranged in use to detect light from theoptical pulses of the second wavelength reflected back along the opticalfiber and to determine any one or more of an acoustic, vibration,temperature or other parameter that perturbs the path length of theoptical fiber in dependence on the reflected light; wherein the opticalfiber is provided with first reflector portions arranged to reflect atleast a portion of signals of the first wavelength in a first section ofthe optical fiber, and the with second reflector portions arranged toreflect at least a portion of signals of the second wavelength in asecond section of the optical fiber.
 10. A system according to claim 9,wherein the first reflector portions are spaced apart from each otherdifferently to the second reflector portions, whereby to providedifferent spatial sensing resolutions in the first and second sectionsof the optical fiber.
 11. An optical fiber distributed sensor systemaccording to claim 9, wherein the system is an optical fiber distributedacoustic sensor system arranged to sense acoustic signals incident uponthe optical fiber.
 12. An optical fiber distributed sensor systemaccording to claim 9, wherein a product of the number of reflectorportions and the average reflectivity of the reflector portions is 0.1or less.
 13. An optical fiber distributed sensor system according toclaim 9, wherein the first section of the optical fiber and the secondsection of the optical fiber are different portions of the opticalfiber.
 14. An optical fiber distributed sensor system according to claim9, wherein the first section of the optical fiber and the second sectionof the optical fiber are overlapping portions of the optical fiber. 15.An optical fiber distributed sensing system, comprising: an opticalfiber deployable in an environment to be sensed, the optical fiberhaving reflector portions regularly distributed in at least a firstregion thereof and having a first spacing therebetween; an opticalsignal source arranged in use to input optical pulses into the opticalfiber; and sensing apparatus arranged in use to detect light from theoptical pulses reflected back along the optical fiber and to determineany one or more of an acoustic, vibration, temperature or otherparameter that perturbs the path length of the optical fiber independence on the reflected light; wherein the reflectivity of thereflector portions alters in dependence on the position of the reflectorportion along the fiber.
 16. A system according to claim 15, wherein thecrosstalk response of the fiber is tuned by varying the reflectivity ofthe reflector portions.
 17. A system according to claim 15, wherein thereflectivity of the reflector portions increases in dependence on: a)the distance along the fiber from the optical signal source; or b) anoptical loss along the fiber from the optical signal source.
 18. Asystem according to claim 15, wherein the reflectivity of the reflectorportions increases: a) in dependence on optical losses of connectors andfeedthroughs within the system; or b) deterministically in accordancewith a mathematical function of the distance along the fiber, whereinthe mathematical function is a monotonic function relating distancealong the fiber to reflectivity of the reflector portions.
 19. A systemaccording to claim 15, wherein the reflectivity of the reflectorportions are tuned to compensate for optical losses in the fibre and soequalise, or otherwise tune, the signal to noise performance, as well astune crosstalk, along the fibre.
 20. A system according to claim 15,wherein a crosstalk contribution from regions along the optical fiberwith a large acoustic signal of low importance can be negated byproviding low reflectivity reflectors or no reflectors along thatsection.